Astronomy & Astrophysics manuscript no. phiperph c ESO 2018 May 25, 2018 Spectral and spatial imaging of the Be+sdO binary ' Persei ? D. Mourard1, J. D. Monnier2, A. Meilland1, D. Gies3, F. Millour1, M. Benisty4, X. Che2, E. D. Grundstrom5, R. Ligi1, G. Schaefer6, F. Baron2; 3, S. Kraus7, M. Zhao8, E. Pedretti9, P. Berio1, J.M. Clausse1, N. Nardetto1, K. Perraut4, A. Spang1, P. Stee1, I. Tallon-Bosc10, H. McAlister3; 6, T. ten Brummelaar6, S.T. Ridgway11, J. Sturmann6, L. Sturmann6, N. Turner6, and C. Farrington6 1 Laboratoire Lagrange, UMR 7293 UNS-CNRS-OCA, Boulevard de l’Observatoire, B.P. 4229 F, 06304 Nice Cedex 4, France e-mail: [email protected] 2 Department of Astronomy, University of Michigan, Ann Arbor, MI 48109, USA 3 CHARA, Georgia State University, P.O. Box 3969, Atlanta, GA 30302-3969, USA 4 Univ. Grenoble Alpes, IPAG, F-38000 Grenoble, France, CNRS, IPAG, F-38000 Grenoble, France 5 Physics and Astronomy Department, Vanderbilt University, 6301 Stevenson Center, Nashville, TN 37235, USA 6 CHARA Array, Mount Wilson Observatory, Mount Wilson, CA 91023, USA 7 University of Exeter, School of Physics, Stocker Road, Exeter EX4 4QL, UK 8 Department of Astronomy and Astrophysics, Pennsylvania State University, University Park, PA 16802, USA 9 Heriot-Watt University, Edinburgh, Scotland EH4 4AS, UK 10 Université de Lyon, 69003 Lyon, France; Université Lyon 1, Observatoire de Lyon, 9 avenue Charles André, 69230 Saint Genis Laval; CNRS, UMR 5574, Centre de Recherche Astrophysique de Lyon; Ecole Normale Supérieure, 69007 Lyon, France 11 National Optical Astronomy Observatory, PO Box 26732, Tucson, AZ 85726, USA ; ABSTRACT Aims. The rapidly rotating Be star ' Persei was spun up by mass and angular momentum transfer from a now stripped-down, hot subdwarf companion. Here we present the first high angular resolution images of ' Persei made possible by new capabilities in long- baseline interferometry at near-IR and visible wavelengths. We analyzed these images to search for the companion, to determine the binary orbit, stellar masses, and fluxes, and to examine the geometrical and kinematical properties of the outflowing disk surrounding the Be star. Methods. We observed ' Persei with the MIRC and VEGA instruments of the CHARA Array. MIRC was operated in six-telescope mode, whereas VEGA was configured in four-telescope mode with a change of quadruplets of telescopes during two nights to improve the (u; v) plane coverage. Additional MIRC-only observations were performed to track the orbital motion of the companion, and these were fit together with new and existing radial velocity measurements of both stars to derive the complete orbital elements and distance. We also used the MIRC data to reconstruct an image of the Be disk in the near-IR H-band. VEGA visible broadband and spectro- interferometric Hα observations were fit with analytical models for the Be star and disk, and image reconstruction was performed on the spectrally resolved Hα emission line data. Results. The hot subdwarf companion is clearly detected in the near-IR data at each epoch of observation with a flux contribution of 1.5% in the H band, and restricted fits indicate that its flux contribution rises to 3.3% in the visible. A new binary orbital solution is determined by combining the astrometric and radial velocity measurements. The derived stellar masses are 9:6±0:3M and 1:2±0:2M for the Be primary and subdwarf secondary, respectively. The inferred distance (186 ± 3 pc), kinematical properties, and evolutionary state are consistent with membership of ' Persei in the α Per cluster. From the cluster age we deduce significant constraints on the initial masses and evolutionary mass transfer processes that transformed the ' Persei binary system. The interferometric data place strong constraints on the Be disk elongation, orientation, and kinematics, and the disk angular momentum vector is coaligned with and has the same sense of rotation as the orbital angular momentum vector. The VEGA visible continuum data indicate an elongated shape for the Be star itself, due to the combined effects of rapid rotation, partial obscuration of the photosphere by the circumstellar disk, and flux from the bright inner disk. Key words. stars: individual: ' Persei - stars: binaries arXiv:1503.03423v1 [astro-ph.SR] 11 Mar 2015 1. Introduction spectral classification refers to the presence of Balmer emission lines that form in their outflowing disks. Such emission appears For about 30 years, Be stars have been favorite targets of op- in B-stars with equatorial velocities above approximately 75% tical interferometers (Thom et al. 1986; Mourard et al. 1989), of the critical rate, and processes related to nonradial pulsation and they are important objects to study the role of rotation in and/or small-scale magnetic fields probably aid mass loss into the link between a star and its close environment. Be stars are the disk (Rivinius et al. 2013). In some circumstances, these rapidly rotating, main-sequence B-stars that eject gas into cir- processes may produce high-energy X-rays. The Be star γ Cas- cumstellar disks (Rivinius et al. 2013). The “e” suffix in their siopeiae, for example, exhibits thermal X-ray emission. In a re- ? Based on observations with MIRC-6T and VEGA-4T instruments cent series of two papers on γ Cas, Smith et al. (2012) and Stee on the CHARA Array et al. (2012) have reviewed the most remarkable features of this Article number, page 1 of 16 star: X-ray activity, magnetic field, critical rotation, Keplerian by Thaller et al. (1995), who showed that the spectrum appears rotation, disk elongation, and binarity. These papers have also similar to that of the sdO6 subdwarf star HD 49798. Božic´ shown once more that a multiwavelength and multitechnique ap- et al. (1995) presented a comprehensive review of the avail- proach is key to making progress in understanding the physics of able photometric and radial velocity data, and they proposed an these complex systems. orbital solution with a period P = 126:6731 days, an epoch The origin of the rapid spin of Be stars is unknown. Most Be of Be star superior conjunction Tsc = HJD2435046:73 (HJD stars appear to be somewhat evolved objects (McSwain & Gies = heliocentric Julian Date), and revised masses: Mp between 2005; Zorec et al. 2005), so that their fast spin represents pro- 16 and 22 M and Ms between 1.7 and 2.2 M . They clas- cesses that occur well into their main-sequence lives. One pos- sified the primary star as B0.5e. The companion was finally sibility is that B-stars spin up as they conclude core H-burning confirmed by Gies et al. (1998) thanks to high-resolution UV due to the redistribution of internal angular momentum (Ekström spectroscopy with the Hubble Space Telescope. The subdwarf- et al. 2008; Granada et al. 2013; Granada & Haemmerlé 2014). to-Be flux ratio is 0:165 ± 0:006 (resp. 0:154 ± 0:009) for the A second possibility is that Be stars were spun up by mass and 1374Å region (resp. 1647Å). Moreover, they derived a double- angular momentum transfer in a binary (Pols et al. 1991; de lined solution for the radial velocity curve that yields masses of Mink et al. 2013). In this case, the companion would lose most Mp = 9:3 ± 0:3 M and Ms = 1:14 ± 0:04 M . The lower mass of its envelope and would appear as a hot, stripped-down He-star estimates with respect to the results from Božic´ et al. (1995) are remnant. The Be binary star ' Persei represents the first detec- explained by a much smaller secondary semiamplitude in the tion of a Be star with a hot, faint companion (Poeckert 1981; RV measurements, which leads to a smaller projected semima- Thaller et al. 1995; Gies et al. 1998), and it is one of three such jor axis and thus lower masses. The subdwarf effective tem- systems known at present (Peters et al. 2013). Clearly it is im- perature is Teff = 53000 ± 3000K and the surface gravity is portant to study such Be binaries to determine the physical prop- log g = 4:2 ± 0:1. erties of the stars at the conclusion of their transformative inter- action. Through the study of the helium lines, Štefl et al. (2000) ar- This paper is an analysis of the results obtained on ' Persei gued for an origin of the emission in the outer parts of the disk thanks to a combined visible and infrared interferometric cam- surrounding the primary star. Their observations agree with a paign with the instruments VEGA (Mourard et al. 2009) and scenario where the outer parts of an axisymmetric disk are illu- MIRC (Monnier et al. 2008) on the CHARA Array (ten Brum- minated by the radiation of the secondary. They also favor in- melaar et al. 2005) and is complemented with a new analysis of homogeneities in the global density pattern of the inner regions a collection of radial velocity measurements. In Sect. 2 we sum- as evidenced by the emission line asymmetry. The model of the marize the known facts on ' Persei relevant to this study, and we Fe II 5317Å and He I 6678Å and 5876Å emission lines of ' compile radial velocity measurements in Sect. 3. The journal of Persei proposed by Hummel & Štefl (2001) yields the size and interferometric observations is presented in Sect. 4, and MIRC shape of the excitation region in the circumprimary disk. The data are presented in Sect. 5 along with a global astrometric and Fe II emission originates within 12 stellar radii in an axisym- radial velocity analysis. The VEGA data and analysis are pre- metric disk around the primary, whereas the He I emission is best sented in Sect.